I wouldn't say Carthage always leaves North Africa empty. I think depends form campaign to campaign.
I one of my campaigns they had most of the cities in Africa fortified with 3 stacks of elites in and around Kart-Hadast, 1 next to Lepkis, 1/2 stack in Ippone.And that was with money script.
In same game they had stack in Sardinia , Bocchoris, 2 in spain, and 1/2 in Corsica.
In another campaign , however they had in all of North Africa total of 1 stack and 1-3 units/per town as garrisons.
It's true that more often AI leaves inner towns with lower garrisons, then stronger ones. It goes for all factions.
+++ In either of campaigns i was not bordering them.
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